This invention relates to the design and manufacture of products having elongated flexible shafts, and more generally to the location of the so-called “neutral plane” in an elongated shaft taking into account irregular characteristics in the shaft geometry, including wall thickness, shaft roundness, longitudinal straightness, material variation, and welded seams. This invention has particular reference to golf club shafts, that typically are longitudinally tapered from a larger “butt” end to a smaller “tip” end and are composed of composite material such as carbon, fiberglass or other fibers embedded in epoxy and rolled or wrapped into an elongated tube and subsequently cured, but the invention may be applicable to shafts composed of other materials as well. The illustrative embodiment is described in reference to tubular composite shafts as the primary area of interest, but the invention is not limited to composite shafts and can be used for metal shafts as well.
It is known that composite shafts, whether used for golf clubs, as fishing rods or for other purposes, have irregularities in their structures that produce variations in bending and other properties in the shafts. These irregularities result in variations in swing properties in different planes that extend longitudinally of the shaft. For example, a greater degree of longitudinal stiffness may exist in one plane of bending, making that plane the plane of greatest resistance to bending, or lowest flexibility. In addition to stiffness, other characteristics may be affected as well, including torsional twisting, vibration and other properties that will affect the shaft's performance in service use.
The effects of such variations are matters of concern to manufacturers and assemblers of products that use the shafts. In golf clubs, in particular, in which normal use involves high-speed loading and bending of the shaft followed by impact of a club head on the shaft with a golf ball and subsequent follow through in completion of the golf stroke, balancing of the club with respect to lateral bending, torsional twisting, vibration and stiffness is of critical importance. For these reasons, manufacturers have developed a variety of methods and apparatus for finding what often is referred to as the “spine” or the “seam” of the shaft so each manufacturer can mount its club heads in the angular position on the tip ends of their shafts that they regard as the optimum orientation for their clubs.
Apparatus and methods that have been developed for these purposes include those disclosed in U.S. Pat. No. 4,958,834, which also contains a lengthy background section referring to other early patents in this field. The '834 patent discloses in FIG. 7 a locator fixture in which the butt end of a shaft is held in a set of bearings while the shaft is manually deflected and rotated to detect the seam or spine by feel. This patent then recommends locating the seam at the rear or trailing side of the shaft for maximum driving distance, or alternatively at the front of the shaft.
More recent apparatus and methods are shown and described in U.S. Pat. Nos. 6,183,275, 6,543,125 and 6,250,168. In U.S. Pat. No. 6,183,275, relatively complex mechanisms are provided for claimed precision in locating the seam in either a composite shaft or butt-welded metal shaft. In one method (in FIG. 3), the user places the end of the shaft in rotatable bearing rings, attaches a weight near the midpoint and then rotates the shaft while observing gauge readings indicating the highest deflection. This patent recommends placing the club head so that the seam is on the front side of the shaft. In U.S. Pat. No. 6,543,125, a preferred orientation, or “preferred oscillation plane,” is determined by measuring the oscillation of a shaft when a horizontal impulse is applied, and then analyzing the oscillation or vibration to select the preferred plane. In U.S. Pat. No. 6,250,168, a mechanism is proposed for simply applying a load to the tip end of a shaft while the butt is held stationary but freely rotatable about the longitudinal axis of the shaft. In a somewhat similar approach, U.S. Pat. No. 5,040,279 discloses a method and apparatus that finds a vertically oscillating plane using a cantilever beam frequency set up.
All of the foregoing apparatus and methods, and others of a similar nature, attempt to locate the most favorable attitude or orientation of the golf club shaft through a variety of different approaches, all of which are effective to some degree but are limited by the methodology they employ and the inability of that methodology to analyze the shaft under all of the conditions that actually are encountered during use of the golf club.
The general objective of this invention is to provide a method and apparatus that can be used quickly and effectively to locate the neutral plane of each shaft that will enable the golf club manufacturer to optimize the performance of the assembled club by reducing the negative effects that are imparted to the club head during the golf stroke. For purposes of this disclosure, this plane sometimes will be referred to as the “neutral plane” rather than the spine or seam of the shaft, and the process may be referred to as “neutralizing” the shaft.